package tcu.se.testing.junit5;

public class Reversion{
    String reverse(String s) {
        if (s == null || s.isEmpty()) {
            throw new IllegalArgumentException("不能为空");
        }
        if (!s.matches("[0-9|a-z|A-Z]+")) {
            throw new IllegalArgumentException("必须为数字或字母");
        }
        int length = s.length();
        if (length == 1) {
            return s;
        }
        String lastChar = s.substring(length - 1);
        return lastChar + reverse(s.substring(0, length - 1));
    }
}